As an image forming apparatus for color printing, there is known, for example, a full-color copying machine. In regard to a plurality of output color components composed of cyan (C), magenta (M), yellow (Y) and black (Bk), the full-color copying machine sequentially forms images of the respective color components on a print surface for printing. An image forming system of the full-color copying machine is an electronic photography system by laser beams, for example. In the electronic photography system, a half tone expression is realized by controlling emission of laser beams with a signal a pulse width of which is modulated in response to an image signal. Incidentally in the image forming apparatus like the full-color copying machine, density or gradation characteristics of an image to be formed possibly changes due to a change of an environment where the image forming apparatus is provided or a change with time of apparatus elements such as a photoreceptor or developer. This possibly causes deterioration or instability of an image quality. A so-called calibration is performed for preventing this. The calibration is to form a predetermined patch pattern, for example, on a photosensitive drum or print medium for correcting density or gradation in the printing of image data of the apparatus based upon the density read from the patch pattern. The print medium where the patch pattern (patch-shaped pattern) is thus formed is also called “test print”.
According to the conventional calibration, however, in the image forming apparatus, it is not assumed to use a cartridge type toner supplying mechanism which does not have a stirring mechanism as a toner supplying mechanism. Therefore, problematic calibration is possibly carried out.
According to the cartridge type toner supplying mechanism which does not have the stirring mechanism, upon continuously performing a print primarily using the toner at the same position, the toner is biased and the toner can not be supplied for print region, causing a partial toner-low state. A single toner remaining amount detecting sensor is generally mounted on a cartridge due to restrictions by cost, but it can not cover detections of state of toner state at all the positions of the print region. As a result, in a case where the partial toner-low state occurs at a distance from the sensor, even the toner-low can not be determined.
In addition, upon forming a patch pattern for calibration at a position where the partial toner-low state thus occurs, formation of the patch pattern can not be performed with stable density. Performing corrections by gamma correction data obtained by calibration using data of the patch pattern which is formed with such partially unstable density for both of image data formed at the position where the partial toner-low state occurs and image data formed at the position where the partial toner-low state does not occur causes image deterioration. That is to say, when the calibration is performed in this state, the gradation is not appropriately outputted even at a print position where the toner sufficiently exists, so that the image quality is worse than before calibration.
As a system for preventing occurrence of an image defective due to toner bias, there is a system in which a plurality of pairs of electrodes are arranged for detecting a remaining amount of developers accommodated in a developer vessel and the toner is stirred when the detected toner amount differs (Japanese Patent Laid-Open No. 2001-290356).
[Citation List]
[Patent Literature]
[PTL1] Japanese Patent Laid-Open No. 2001-290356